Steam generator with reheat temperature regulation



Jan. 23, 1958 R, B. COVELL ETAL 3,364,903

STEAMGENERATOR WITH REHEAT TEMPERATURE REGULATION Filed Sept. 8, 1966 f X in EUSSEL 5. Col/ELL VV/LL MM H. 61.4 Yro/v Jfl 205527 A. KANE INVENTOR.

United States Patent 3,364,903 STEAM GENERATOR WITH REHEAT TEMPERATURE REGULATION Russell B. Covei], Granby, William H. Clayton, Jr.,

Windsor, and Robert A. Kane, Hazardville, Conn., assignors to Combustion Engineering, Inc., Windsor, Conn., a corporation of Delaware Filed Sept. 8, 1966, Ser. No. 578,035 11 Claims. (Cl. 122-479) This invention relates to supercritical pressure reheat steam generators for variable load operation and in particular to a method and apparatus for affecting the reheated steam temperature.

In a reheat steam generator high pressure fluid passes through the primary circuits including the economizer, waterwalls, and superheaters. Low pressure steam, being reheated after passage through a portion of a turbine passes through the steam reheating surface. Design considerations of the steam generator generally dictate that at least a major portion of the reheater be placed at a location remote from the intense radiation of the furnace. Reasons for this include the need for a low temperature heat sink, poor conductance of low pressure steam with resultant high tubing metal temperature, and the absence of steam flow through the reheater during portions of the unit startup. Accordingly, this surface is generally placed in the flue which conducts the combustion products from the furnace.

It is desirable to maintain the temperature of this reheated steam as close to its maximum value as possible during operation at any load. A low temperature of the reheated steam leads to excessive moisture problems in the low pressure turbine blading and to a general decrease in cycle efficiency. It is therefore important to regulate the reheated steam temperature to this maximum value throughout the load range.

As load is decreased on a steam generator, a higher percentage of heat is inherently absorbed in the furnace by radiant heat while a lesser percentage is absorbed by the convection surface located in the flue. Therefore as load is decreased, the heat absorbed by the reheater from the gas tends to decrease in relation to the heat absorbed by the primary section in the furnace, and increasing amounts of heat must be added to the reheated steam flow if its steam temperature is to be maintained. This condition is aggravated by the normal steam cycle characteristic in which the steam to be reheated is returned to the boiler at lower temperatures during low load operation, this being due to the turbine and cycle characteristics. Therefore not only does the inherent characteristics of a steam generator tend to reduce heat absorbed by the reheater, but the lower temperature inlet steam obviously requires that a greater percentage of the total heat be added to the reheat steam to obtain the same reheater outlet steam temperature.

Heat exchangers between the primary and reheat steam have been used to affect and regulate the temperature of the reheated steam. The primary flow side of these heat exchangers, however, has been in such a location that the flow through the heat exchanger inherently tends to increase with increased load while it decreases with decreased load. This, of course, is the reverse of the required characteristic Where the reheat steam must be heated more at low loads. Therefore a throttling control imposed on such a heat exchanger, being the opposite of the natural characteristic, makes the flow very diflicult to control. Also these heat exchangers are located such that the effluent from the primary fluid side of the heat exchanger passes to the superheater, the temperature of the fluid leaving the section upstream of the heat exchangers 3,364,993 Patented Jan. 23, 1968 such as the waterwalls must be increased to maintain the same superheater temperature.

In our invention primary fluid is withdrawn from the through-flow path at a location downstream of the waterwall section but upstream of he superheater. This fluid is passed in heat exchange relationship with the reheater steam flow and then returned to the primary flow path at a location upstream of the waterwalls.

It is an object of our invention to provide a reheat steam generator with improved reheat temperature characteristics and control.

It is a further object to provide a method of reheating steam in a vapor generator in such a manner as to obtain an improved reheat temperature characteristic and temperature regulation throughout a load range.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawings wherein:

The illustration is a schematic diagram of a side elevation of a reheat steam generator illustrating our invention.

A through-flow of water is established through feed pipe 2 by a feed pump (not shown) and through economizer 4. This Water then passes to the mixing vessel 6 and from there through downcomer 8 and recirculating pump 10. The water is distributed through waterwall inlet headers 12 to tubes 14 lining the walls of furnace 16.

The water in passing upwardly through tubes 14 absorbs heat from gases within the furnace 16 and is generally in the form of steam when it reaches the waterwall outlet header 18. The through-flow of steam passes through steam pipe 20 to the steam cooled Wall inlet header 22. From this point the steam passes up through tubes 24 lining the walls of the portion of the flue 26. The steam flow through these walls is collected in steam cooled wall outlet header 28.

The through-flow then passes through steam pipe 30 and boiled throttle valve 32 to the superheating surface 34. This superheating surface is comprised of a plurality of parallel tubes supported within the flue 26 of the vapor generator. These tubes are heated partially by radiation, directly from the furnace and partially by convection due to flue gases passing over the surface. The superheated steam then passes outwardly to a steam turbine through superheater outlet pipe 36.

Steam from the turbine is returned to the steam generator through cold reheat line 38 to be reheated. This cold reheat steam flow passes through the heated side 40 of the heat exchanger 42. The steam is conveyed through reheater inlet pipe 44 to the reheater heating surface 46. This heating surface comprises a plurality of parallel tubes supported in flue 26. Heating by means of this surface is accomplished primarily by convection. The reheat steam passes out through the hot reheat line 48 to a reheat turbine.

Fuel is supplied through burners 50, and air is supplied in the same general area. Combustion occurs in furnace 16 with the combustion products passing upwardly through the furnace to flue 26. These combustion products then pass over the surfaces of the superheater 34, the reheater 46 and the economizer 4. The gases then pass outwardly to discharge through an air heater (not shown).

A first portion of the through-flow is withdrawn from the through-flow circuit at header 1%. Flow is conveyed through heat exchange supply pipe 52 and regulating 3 valve 54 to the heating side 56 of the heat exchanger 42. The flow is then passed through heat exchange return line 58 and check valve 60 to the mixing vessel 6. The temperature of the fluid leaving the furnace wall tubing 14 and entering the header 18 is in the order of 780 to 800 F. throughout the load range of the vapor generator. This first portion of fluid which is withdrawn and passed through the heat exchanger 42, therefore, is in the order of 790 F. regardless of the load at which the steam generator is operating. On the other hand, the temperature of the steam to be reheated and being supplied through cold reheat line 38 varies from 550 F. at full load down to 460 F., at half load. It can, therefore, be seen that heat exchanger 42 transfers heat from the withdrawn first portion flowing on the heating side 56 to the reheat steam flowing through the heated side 40. It can also be seen that the temperature difference increases with decrease in load of the vapor generator.

Also of importance is the flow characteristic through the heating side 56 of the heat exchanger 42. Circulating pump floats on the system in such a manner that the recirculation through line 52 is increased as load decreases absent manipulation of regulating valve 54. This principle is discussed and illustrated by US. Patent 3,135,252 to Willburt W. Schroedter. This increase in flow through the heated side 56 of the heat exchanger increases the heat transfer to the reheat steam. This is due only slightly to the improved heat transfer co-eflicient. The primary reason of the increased transfer is the fact that a high flow through the heat exchanger results in less temperature reduction as the fluid passes through the heat exchanger. Therefore towards the outlet of the heat exchanger the heating fluid tends to stay at a high temperature thereby increasing the temperature head available for heat transfer.

Therefore it can be seen that with regulating valve 54 omitted or retained in a wide open position, that this unit will operate to improve the reheat temperature characteristic by correcting deficiencies in reheat temperature at low loads, with there being more correction in the lower loads where such increased correction is required. While this system can be manually controlled the illustrated embodiment also includes controls to automatically regulate the reheat steam temperature to the desired value.

Temperature transmitter 62 senses the temperature of the steam leaving the reheater surface 46. The temperature indicating signal is passed through control line 64 to set point 66 where it is compared with a set point signal 68 which is representative of the desired temperature. After these two signals are compared an error signal is passed through control line 70 to valve operator 72 which operates to regulate throttling valve 54. The valve 54 is regulated to adjust the flow through the heating side 56 of the heat exchanger and to, accordingly, control the temperature leaving the reheating surface 46 to the desired value. The natural characteristic of primary flow through the heat exchanger being in accordance with the heat transfer characteristics, provides for good regulation by control valve 54.

While reheated steam temperature can be controlled by this circuit, the requirement of throttling valve 54 in response to reheat temperature interferes with the basic reason for installing the circulating pump as described in US. Patent 3,135,252 to Willburt W. Schroedter. Therefore to obtain sufficient recirculation to properly protect the water tubes 14, recirculating line 74 is located so as to withdraw a second portion of the through-flow from the steam cooled wall outlet header 28. This flow passes through check valve 76 and is returned to the mixing vessel 6 at a location upstream of the furnace wall tubing 14. The heating surface of the steam cooled walls 24 is intermediate furnace wall outlet header 18 where the first portion is withdrawn and steam cooled wall outlet header 28 from which the second portion is withdrawn. By so locating these two recirculating lines, more pressure drop is available between header 18 and mixing vessel 6 for the heat exchanger circuit than for the ordinary recirculating line 74. This is due to the pressure drop of the fluid flowing from header 18 to header 28. This provides pressure drop to overcome the resistance of the tubing in the heat exchanger 42. This would also provide for recirculation through the heat exchanger even though the steam generator were operating at a load such that the head supplied by circulating pump 10 was not quite sufficient to induce recirculation from header 28.

Since the fluid passing through heat exchanger 42 is not passed on to the superheater but rather returned upstream of the waterwalls, the temperature of the fluid leaving the waterwall need not be increased to provide this heating capacity. The steam temperature leaving the waterwalls is exactly the same as it is entering the next section. The steam leaving the heat exchanger 42 at a relatively low temperature is returned to a location upstream of the furnace wall tubes 14 thereby decreasing the temperature of the fluid entering the waterwalls. Where the waterwall tubes are operating at near maximum design temperature, the advantage of this mode of operation over a serially located heat exchanger is apparent.

While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our invention.

What we claim is:

1. A steam generator for variable load operation having: a furnace; burners for introduction of fuel into said furnace for the combustion of the fuel and the evolution of combustion products; a flue for the conveyance of combustion products from said furnace; a first heating surface lining the walls of said furnace; a second steam heating surface located in said flue; means for establishing a through-flow of water serially through said first and second heating surfaces with said first heating surface upstream of said second heating surface with relation to said through-flow; means for conveying steam from said second heating surface to a point of use; steam reheating surface located in said flue; means for establishing a flow of steam to be reheated through said steam reheating surface; a heat exchanger having a heated side and a heating side; means for passing the flow of steam to be reheated through the heated side of said heat exchanger at a location upstream of said steam reheating surface with respect to steam flow; means for withdrawing a first portion of the through-flow from a location intermediate said first and second heating surfaces and passing said first portion through the heating side of said heat exchanger; and means for returning said first portion from said heat exchanger to a location in the through-flow path upstream with respect to the through-flow of said first heating surface.

2. An apparatus as in claim 1 wherein said steam reheating surface is located downstream of said second heating surface with respect to the flow of combustion products.

3. An apparatus as in claim 1 having also means for regulating the quantity of said first portion withdrawn from the through-flow path.

4. An apparatus as in claim 3 having also means for withdrawing a second portion of the through-flow from a location intermediate said first and second heating surfaces; and means for returning said second portion directly to a location upstream of said first heating surface with respect to the through-flow.

5. An apparatus as in claim 4 wherein said means for returning said first portion and said second portion includes a free-running centrifugal pump at a location such that the flow through the pump is commn to said first and second flow portions.

6. An apparatus as in claim 4 having also: a third heating surface intermediate said first and second heating surfaces with respect to the through-flow; said first portion being withdrawn from a location upstream of said third heating surface; and said second portion being Withdrawn from a location downstream of said third heating surface.

7. An apparatus as in claim 6 wherein said third heating surface comprises tubing lining the walls of at least a portion of said flue.

8. An apparatus as in claim 6 having also a fourth heating surface located in said flue at a location downstream of said steam reheating surface with respect to gas flow; said fourth heating surface being upstream of said first heating surface with respect to through-flow.

9. An apparatus as in claim 3 having also: means for determining the temperature of the steam leaving said steam reheating surface; said means to regulate the flow of said first portion being responsive to said means for determining temperature.

10. In a steam generator having first, second, and third heating surfaces the method of operation comprising: establishing a primary flow serially through said first and second heating surfaces; recirculating a portion of said primary flow through said first heating surface from a location intermediate said first and second heating surface to a location immediately upstream of said first heating surface; burning fuel in radiant heat exchange with the flow in said first heating surface and establishing flow of hot combustion gases; passing the hot combustion gases in heat exchange relationship with the primary fiow passing through said second heating surface; establishing a reheat flow and passing said reheat flow through said third heating surface in heat exchange relation with said combustion gases; passing the recirculating portion of the primary flow in heat exchange relation with said reheat flow at a location upstream of said third heating surface with respect to reheat flow; sensing the temperature of the reheat flow leaving said third heating surface; and regulating the quantity of the recirculating portion of the primary flow in response to the sensed temperature to control the temperature of the reheat flow to a desired value.

11. The method as in claim 10 including: increasing the primary flow quantity to increase steam generator output, and simultaneously decreasing the quantity of the recirculated portion of the primary flow.

References Cited UNITED STATES PATENTS 2,968,156 1/1961 Pacault et a1 -73 3,111,936 11/1963 Brunner 122479 FOREIGN PATENTS 1,087,620 5/ 1961 Germany.

957,024 5/ 1964 Great Britain.

KENNETH W. SPRAGUE, Primary Examiner. 

1. A STEAM GENERATOR FOR VARIABLE LOAD OPERATION HAVING: A FURNACE; BURNERS FOR INTRODUCTION OF FUEL INTO SAID FURNACE FOR THE COMBUSTION OF THE FUEL AND THE EVOLUTION OF COMBUSTION PRODUCTS; A FLUE FOR THE CONVEYANCE OF COMBUSTION PRODUCTS FROM SAID FURNACE; A FIRST HEATING SURFACE LINING THE WALLS OF SAID FURNACE; A SECOND STEAM HEATING SURFACE LOCATED IN SAID FLUE; MEANS FOR ESTABLISHING A THROUGH-FLOW OF WATER SERIALLY THROUGH SAID FIRST AND SECOND HEATING SURFACES WITH SAID FIRST HEATING SURFACE UPSTREAM OF SAID SECOND HEATING SURFACE WITH RELATION TO SAID THROUGH-FLOW; MEANS FOR CONVEYING STEAM FROM SAID SECOND HEATING SURFACE TO A POINT OF USE; STEAM REHEATING SURFACE LOCATED IN SAID FLUE; MEANS FOR ESTABLISHING A FLOW OF STEAM TO BE REHEATED THROUGH SAID STEAM REHEATING SURFACE; A HEAT EXCHANGER HAVING A HEATED SIDE AND A HEATING SIDE; MEANS FOR PASSING THE FLOW OF STEAM TO BE HEATING THROUGH THE HEATED SIDE OF SAID HEAT EXCHANGER AT A LOCATION UPSTREAM OF SAID STEAM REHEATING SURFACE WITH RESPECT TO STEAM FLOW; MEANS FOR WITHDRAWING A FIRST PORTION OF THE THROUGH-FLOW FROM A LOCATION INTERMEDIATE SAID FIRST AND SECOND HEATING SURFACES SIDE OF SAID HEAT EXCHANGER; AND THROUGH THE HEATING SIDE OF SAID HEAT EXCHANGER; AND MEANS FOR RETURNING SAID FIRST PORTION FROM SAID HEAT EXCHANGER TO A LOCATION IN THE THROUGH-FLOW PATH UPSTREAM WITH RESPECT TO THE THROUGH-FLOW OF SAID FIRST HEATING SURFACE. 